1. Field of the Invention
The present invention relates to materials which reduce fog levels or increase the sensitometric speed in photothermographic imaging elements. These elements comprise a photosensitive silver halide, silver salt oxidizing agent, and reducing agent for silver ion in a binder. The antifoggants of the present invention comprise hydrobromic acid salts of nitrogen containing heterocyclic ring compounds which are further associated with a pair of bromine atoms.
2. Background of the Art
Silver halide photothermographic imaging materials, often referred to as "dry silver" compositions because no liquid development is necessary to produce the final image, have been known in the art for many years. These imaging materials basically comprise a light insensitive, reducible silver source, a light sensitive material which generates silver when irradiated, and a reducing agent for the silver source. The light sensitive material is generally photographic silver halide which must be in catalytic proximity to the light insensitive silver source. Catalytic proximity is an intimate physical association of these two materials so that when silver specks or nuclei are generated by the irradiation or light exposure of the photographic silver halide, those nuclei are able to catalyze the reduction of the silver source by the reducing agent. It has been long understood that silver is a catalyst for the reduction of silver ions and the silver-generating light sensitive silver halide catalyst progenitor may be placed into catalytic proximity with the silver source in a number of different fashions, such as partial metathesis of the silver source with a halogen-containing source (e.g., U.S. Pat. No. 3,457,075), coprecipitation of the silver halide and silver source material (e.g., U.S. Pat. No. 3,839,049), and any other method which intimately associates the silver halide and the silver source.
The silver source used in this area of technology is a material which contains silver ions. The earliest and still preferred source comprises silver salts of long chain carboxylic acids, usually of from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of like molecular weight have been primarily used. Salts of other organic acids or other organic materials such as silver imidazolates have been proposed, and U.S. Pat. No. 4,260,677 discloses the use of complexes of inorganic or organic silver salts as image source materials.
In both photographic and photothermographic emulsions, exposure of the silver halide to light produces small clusters of silver atoms. The imagewise distribution of these clusters is known in the art as the latent image. This latent image generally is not visible by ordinary means and the light sensitive article must be further processed in order to produce a visual image. The visual image is produced by the catalytic reduction of silver ions which are in catalytic proximity to the specks of the latent image.
U.S. Pat. No. 4,460,681 discloses a color photo-thermographic element in which color forming layers are separated by barrier layers to prevent migration of components between layers which would reduce the color separation.
U.S. Pat. No. 4,594,307 discloses a thermal diffusion transfer photothermographic element in which individual color sheets are used to provide colors. Multiple color images are formed by the use of multiple sheets of different colors.
Photothermographic emulsions, in a manner similar to photographic emulsions and other light sensitive systems, tend to suffer from fog. This spurious image density which appears in non-developmentally sensitized areas of the element. This is often reported in sensitometric results as D.sub.min. This problem is also related to certain stability factors in the photosensitive elements where fog increases upon storage of the photosensitive element.
U.S. Pat. No. 4,212,937 describes the use of a nitrogen-containing organic base in combination with a halogen molecule or an organic haloamide to improve storage stability and sensitivity.
Japanese Patent Kokai JA 61-129642 published June 17, 1986 describes the use of halogenated compounds to reduce fog in color-forming photothermographic emulsions. These compounds include acetophenones including phenyl-(alpha,alpha-dibromobenzyl)-ketone.
U.S. Pat. No. 4,152,160 describes the use of carboxylic acids including benzoic acids and phthalic acids in photothermographic elements. These acids are used as antifoggants. The benzoic acids have the general formula ##STR1## with the various substituents selected from amongst hydrogen, cyano, nitro and halogen.
U.S. Pat. No. 3,589,903 describes the use of small amounts of mercuric ion in photothermographic silver halide emulsions to improve speed and aging stability.
U.S. Pat. No. 4,784,939 describes the use of benzoyl acid compounds of a defined formula to reduce fog and to improve the storage stability of silver halide photothermographic emulsions. The addition of halogen molecules to the emulsions are also described as improving fog and stability.